Miche Collapse

I was trying to make Hamelman's ("Bread") Mixed Flour Miche. It was going fine, a nice active starter got the levain going. After mixing with 10 minutes of Rubaud's method, a rest, and some folds, the dough seemed to be developing. After 3 more rounds of folds 30 minutes apart, it seemed pretty strong, especially for ~85% whole grains (15% high gluten, 15% rye, 70% whole wheat